It is common to store water in unpressurized vessels from which water from time to time is drawn by means of pump or gravity flow, particularly where the water supply is intermittent, or where the instantaneous demand exceeds the flow rate of the source of supply. It is particularly desirable, to have a vessel which is sealed against egress or ingress of liquids when potable water is being stored.
Typical water storage tanks that are associated with dwellings and other smaller buildings will have 500 to 1000 liter capacity. A squat cylindrical shape tank is popular for reason of compactness and manufacturability. A representative 500 liter tank might be about 100 cm in diameter and 100 cm high. One approach is to make a one piece tank, such as by blow molding or rotational molding. However, if a whole tank ready for use is shipped, the shipping cost can be an unacceptably high fraction of the total cost of making and delivering the product.
Thus, one marketplace solution has been to have manufacturing sites dispersed around the country, when it is geographically large as is the U.S. However, for lower volume products with high machinery or tooling costs, the net result of long distance shipping or duplication of manufacturing facilities can be that the price of the product to the consumer is higher than it might be if the product could be made so that it could be shipped in more compact form.
One solution has been to form a tank from mating identical half tanks which can be nested for shipment and assembled in proximity to the point of use. See U.S. Pat. No. 5,878,907 of Graf. The tank has a mid-elevation joint which is secured by means of clamps or welding. Another approach used in commerce is to make the tank in the form of a bucket like base which is combined with an associated lid. The bases have sufficient draft to enable nesting for shipment. The present invention is concerned with such types of tanks. Further improvements in design and manufacturing are needed for such kinds of tanks, to reduce manufacturing costs while providing a tank which is good for potable water, which has a joint near the top of the tank that is secure and not overly sensitive to small lid-to-base joint imperfections. The tank also should be suited for quick and easy assembly.
When water storage tanks are used for potable water there can be other needs to be satisfied. Depending on the user and specific application, it may be desirable that the exterior of the tank be colored and not show dirt and grime as will a white tank, that the tank wall will be opaque to inhibit passage of sunlight, that the plastic materials of construction be appropriate for preserving the purity of the water, and that a user may be able to tell, by looking into the tank, what is the cleanliness condition of the tank and water.